Arsenal have launched a full review into the ugly scenes that marred Thursday night's Europa League win against Cologne.
Twenty thousand German supporters went on the rampage at the Emirates before the Gunners came from behind to win 3-1.
Kick-off was delayed by an hour as Cologne fans tried to storm the turnstile gates, attacked stewards and burst through police cordons outside.
Arsenal and Cologne will now face a full scale UEFA investigation with possible sanctions including heavy fines and potential ground closures in the future.
The Gunners said: "We worked in full consultation with police and UEFA officials on the night and in advance of the game and had taken extensive steps in advance to prevent tickets being sold to visiting supporters.
"This included a number of measures including ensuring no tickets were sold via general sale and that no red memberships purchased after the draw were able to be used to get tickets in the home end for this match. We also worked with our colleagues at Cologne to stop supporters travelling without match tickets.
"The 3,000 tickets issued to Cologne fans was in line with competition rules but it is clear many more visiting fans arrived, causing significant congestion and disturbance outside the stadium before kick-off. Many tickets were sold through touts and this is very disappointing.
"We have launched a full review into the circumstances surrounding the game and will ensure any lessons that can be learned are used in the future."